The present disclosure relates in various exemplary embodiments to thermochromic reimageable recording medium, e.g., thermal paper. More particularly, the present disclosure relates to a reimageable recording medium comprising a thermochromic composition that allows a visible image to be formed as desired by the application of heat to selected areas of the medium. The present disclosure also relates to a method for forming an image using such thermochromic reimageable recording medium.
Paper documents are often promptly discarded within a relatively short time after being read. Although paper is inexpensive, the quantity of discarded paper documents is enormous and the disposal of these discarded paper documents raises significant cost and environmental issues. Thus, it is desirable to provide a recording medium that is reusable and/or reimageable.
One method for providing a reimageable recording medium is to provide a recording medium coated with a photochromic material. Photochromic materials change from a colorless to a colored state when exposed to ultraviolet light. Copending application Ser. Nos. 10/835,518 and 10/834,722 are respectively directed to a reimageable recording medium comprising a photochromic material and a method for forming an image using such a reimageable recording medium. Some examples of known photochromic materials include spiropyrans and spiroxazines. Upon exposure to ultraviolet light, the closed ring structures, which exists in a colorless state, open and exhibit a colored state. The formulas below depict the closed and open states of a spiropyran and spiroxazine, respectively.
The above materials return to a colorless state by heating or by exposure to visible light. The stability of the colored state of spiropyrans has been shown to be enhanced by the addition of ZnCl2 to a spiropyran compound in solutions. See “Effects of metal ion complexation of the spiropyran-merocyanine interconversion: development of a thermally stable photo-switch,” James T. C. Wojtyk et al., Chem. Common, 1703 (1998).
Another option is to form a thermochromic recording medium, i.e., thermal paper, by use of a thermochromic material. Thermochromic materials change color upon exposure to heat. U.S. Pat. No. 5,403,702 is directed to an indolino spirobenzo pyran derivative described as being suitable for use as a thermochromic and photochromic material. Other patents and/or published applications disclosing the use of thermochromic materials and/or thermal paper include, for example, U.S. Pat. Nos. 3,561,269; 4,421,560; 4,425,161; 4,523,207; 4,601,588; 4,717,710; 5,112,526; 5,281,570; 5,350,633; 5,350,634; 5,352,649; 5,527,385; 5,558,699; 5,688,592; 5,883,043; 5,932,318; 6,562,755; 6,048,387; and 6,803,344, and U.S. Patent Application Publication Nos. 2003/0002428; 2004/0014396; 2004/0077743; and 2004/0198600.
There is still a need for a thermochromic reimageable recording medium comprising a thermochromic composition that exhibits a suitable color contrast. There is also a need for a thermochromic reimageable recording medium that is relatively. stable in dark conditions or in the absence of high intensity visible light.